The exhaust gas emitted from an internal combustion engine, is a heterogeneous mixture that contains gaseous emissions such as carbon monoxide (“CO”), unburned hydrocarbons (“HC”) and oxides of nitrogen (“NOx”) as well as condensed phase materials (liquids and solids) that constitute particulate matter. Catalyst compositions, typically disposed on catalyst supports or substrates that are disposed within the exhaust system of an internal combustion engine are provided to convert certain or all of these exhaust gas constituents into non-regulated exhaust gas components. For example, exhaust systems for internal combustion engines may include one or more of a precious metal containing oxidation catalyst (“OC”) device for the reduction of CO and excess HC, a selective catalytic reduction catalyst (“SCR”) device for the reduction of NOx and a particulate filter (“PF”) device for the removal of particulate matter.
An exhaust gas treatment technology in use for high levels of particulate matter reduction, the PF device may utilize one of several known exhaust gas filter structures that have displayed effectiveness in removing the particulate matter from the exhaust gas. Such exhaust gas filter structures may include, but are not limited to ceramic honeycomb wall flow filters, wound or packed fiber filters, open cell foams, sintered metal fibers, etc. Ceramic wall flow filters have experienced significant acceptance in automotive applications.
The exhaust gas filter structure is a physical structure for removing particulates from exhaust gas and, as a result, the accumulation of filtered particulates in the exhaust gas filter structure will have the effect of increasing backpressure in the exhaust system that is experienced by, and that must be overcome by, the internal combustion engine. To address backpressure increases caused by the accumulation of exhaust gas particulates in the exhaust gas filter structure, the PF device is periodically cleaned, or regenerated. Regeneration of a PF device in vehicle applications is typically automatic and is controlled by an engine or other suitable controller based on signals generated by engine and exhaust system sensors. The regeneration event involves increasing the temperature of the exhaust gas filter structure of the PF device, typically by heating the engine exhaust gas, to levels that are often above 600° C. in order to burn the accumulated particulates.
One method of generating the exhaust gas temperatures required in the exhaust system for regeneration of exhaust gas filter structure of the PF device is to deliver unburned HC to an OC device disposed upstream of the PF device or to an oxidation catalyst compound disposed in the PF device itself. The HC may be delivered to the exhaust system by direct fuel injection into the exhaust system or may be achieved by “over-fueling of” or “late injection of fuel to” the internal combustion engine. The result is unburned HC mixed with the exhaust gas flowing through the exhaust system that is oxidized by the oxidation catalyst in an exothermic reaction that raises the temperature of the exhaust gas. The heated exhaust gas burns the particulate accumulation in the exhaust gas filter structure of the PF device. The addition of an oxidation catalyst to the exhaust gas filter structure can assist in lowering the oxidation temperature of soot and particulates and thus the regeneration temperatures required. This results in increased durability of the PF device and lower HC requirements for regeneration and, therefore, improved fuel economy for the internal combustion engine. In addition, such an oxidation catalyst applied to the exhaust gas filter structure of the PF device is useful to oxidize any remaining excess HC in the exhaust gas as well as reducing carbon monoxide constituents (“CO”) resulting for the combustion of soot and particulates.
A technology that has been developed to reduce the levels of NOx emissions in exhaust gas produced by internal combustion engines that burn fuel in excess oxygen includes a selective catalytic reduction (“SCR”) device. An SCR catalyst composition in the SCR device preferably contains a zeolite and one or more base metal components such as iron (“Fe”), cobalt (“Co”), copper (“Cu”) or vanadium (“V”) which can operate efficiently to convert NOx constituents in the exhaust gas in the presence of a reductant such as ammonia (‘NH3”). NOx sensors placed at locations upstream and downstream of the SCR device monitor NOx conversion efficiency and information supplied by the sensors is utilized by a reductant control system to determine the quantity of NH3 to be injected into the exhaust system for use by the SCR device.
For exhaust treatment systems utilized particularly in vehicular applications, space, cost and performance requirements have, at times, necessitated the placement of one or more exhaust treatment devices in a single canister or housing. For instance, an SCR device and a PF device may be housed together. In certain situations such as Chassis Certification the SCR device is preferably placed upstream of the PF device in order to reduce the thermal inertia ahead of the SCR so that it can be warmed up rapidly to start NOx reduction as soon as possible. However, the addition of an oxidation catalyst to the exhaust gas filter structure can result in the conversion of excess NH3 exiting the SCR device into NOx resulting in a misleading or unreliable NOx sensor reading by the downstream NOx sensor. Such a reading may result in improper dosing of NH3 by the reductant control system (e.g. the NOx sensor on sensing high NOx (coming from NH3 oxidation) will command for more NH3). This will result in even higher NH3 slip and so forth. Additionally, NH3 is an unregulated gas and its oxidation to a regulated gas like NOx is an undesirable feature of an oxidative PF device. One solution for this problem is to locate the downstream NOx sensor in the space between the substrates of the SCR device and the PF device utilizing a snorkel device that extends into the space and diverts a portion of the exhaust gas exiting the SCR device to a NOx sensor. However, due to uneven distribution of NH3 and NOx concentrations across the exiting face of the SCR device, as well as the limited packaging space between the two substrates, it is difficult to obtain an accurate and reliable NOx reading that is representative of the exhaust gas exiting the SCR device across its entire exit cross section.